If you're trying to figure out the most or least fertile days in your menstrual cycle, you might be tracking your cycle a bit obsessively. You could even be wondering if you can get pregnant right before your period.
And yep, you can: "It is always possible to get pregnant at any point in your cycle," points out Monica McHenry Svets, MD, an ob-gyn at Cleveland Clinic.
However, conceiving three or four days before your period starts is not very likely. That's because most women have their most fertile days around the midpoint of their cycle — at least two weeks before menstruation starts. Still, getting pregnant right before your period is still possible because menstrual cycles can be unpredictable.
"Many factors that we don't know about or have much control over can influence when a woman ovulates and, thus, when she is fertile," notes Dr. Svets. If you're not ready to have a baby right now, protection is a must (otherwise — you're gambling, she adds).
If you're wondering, "Can I get pregnant four days before my period?" the answer is yes. If you're asking if that timeframe is the best time to conceive, the answer is no.
The Average Menstrual Cycle
The average menstrual cycle lasts 28 days, with a usual range of 25 to 36 days. For women with a 28-day cycle, ovulation — or the egg release — usually takes place on day 14 of the menstrual cycle, or about 12 to 14 days before menses — when the bleeding starts, according to the American College of Obstetricians and Gynecologists (ACOG).
So if you have an average cycle and no known fertility problems, you are most likely to conceive up to five days before ovulation through the day of ovulation, according to the ACOG. However, a variety of factors make it challenging to predict these fertile days with precision.
The Irregular Menstrual Cycle
According to the Merck Manual, at least 20 percent of women have irregular cycles, which means the length is shorter or longer than usual. A smaller number (10 to 15 percent) of women have cycles that are exactly 28 days in length, says the same organization. In addition, women can sporadically get their periods late, even if their cycles appear to be normal.
The Ovulation Variable
Many factors need to come together for a successful pregnancy. Among these variables, the sperm and egg need to be healthy and compatible, the uterus needs to be ready for a fertilized egg to successfully implant and sexual intercourse needs to happen during the fertile window.
This timing needs to be matched with ovulation, as the most fertile days precede this event. But when a woman ovulates later than expected, she can be fertile — and get pregnant — right before she expects her period to start.
If you're using fertility awareness either to avoid or achieve pregnancy, it's essential to know when you're ovulating — and there are several ways to determine this. One involves tracking your basal body temperature, which runs lower in the beginning of the menstrual cycle, then rises when ovulation occurs, per the Mayo Clinic. "Take your temperature daily throughout the month and you'll see as much as a one degree rise in temperature around ovulation," says Dr. Svets.
Another method involves assessing cervical mucus, since this body fluid changes in amount, texture and color before ovulation. Dr. Svets says to look for copious mucous around ovulation time (it'll look and feel a little like egg whites).
Women may also use the Standard Days Method, developed by the Institute for Reproductive Health at Georgetown University. This calendar method calculates fertile days based on average cycle length. Ovulation predictor kits that measure the urine level of luteinizing hormone (LH) are also helpful, since LH rises right before ovulation.
Effectiveness of Fertility Awareness
Whether you're looking to have a baby or avoid pregnancy, women benefit from using a combination of fertility awareness methods. However, these are not 100-percent effective at preventing pregnancy.
According to ACOG, flawless use of fertility awareness methods results in fewer than one to five percent of women getting pregnant during the first year. However, with typical use, which involves some incorrect or inconsistent use (we're all human!), 12 to 24 percent of women using these methods will become pregnant in the first year.
Drawbacks of Fertility Awareness
Fertility awareness methods that use calendar calculations or body awareness in order to prevent pregnancy are not as effective other methods — such as birth control pills, condoms, implants or intrauterine devices. So if you're trying to prevent pregnancy, discuss all of your options with your health care provider so you can use a birth control method that's effective and safe.
If you prefer to use fertility awareness as your method of birth control — or if trying to conceive — be sure to learn how to use this correctly to maximize effectiveness. And if you're trying to conceive and you're not getting pregnant, seek the advice or your doctor or nurse practitioner.
The Bottom Line
Because of the unpredictable nature of the menstrual cycle, it's possible to get pregnant three or four days before your period, or even the day before your expected period.
However, the chance of conceiving at this time is very low, and if you find yourself getting pregnant from sex just before your scheduled period, it's much more likely that you ovulated late and conceived just before you thought your period was going to arrive. "The more evidence for ovulation one has, the easier it can be to time intercourse when trying for pregnancy," says Dr. Svets.